refactor: extract shared func CLI download with checksum verification (#132)

Author: creydr

internal/funccli/download.go

@@ -0,0 +1,147 @@
+package funccli
+
+import (
+	"context"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	goruntime "runtime"
+	"strings"
+	"time"
+)
+
+// DownloadOptions configures the binary download behavior.
+type DownloadOptions struct {
+	// HTTPClient is the HTTP client to use for downloading.
+	// If nil, a default client with a 30-second timeout is used.
+	HTTPClient *http.Client
+}
+
+func (o *DownloadOptions) httpClient() *http.Client {
+	if o != nil && o.HTTPClient != nil {
+		return o.HTTPClient
+	}
+	return &http.Client{Timeout: 30 * time.Second}
+}
+
+// AssetName returns the func CLI asset name for the current platform
+// (e.g. "func_linux_amd64").
+func AssetName() string {
+	return fmt.Sprintf("func_%s_%s", goruntime.GOOS, goruntime.GOARCH)
+}
+
+// DownloadAndInstall downloads a func CLI binary from binaryURL, verifies its
+// SHA256 checksum against the checksums.txt at checksumURL, and atomically
+// installs it to installPath. The parent directory of installPath must exist.
+func DownloadAndInstall(ctx context.Context, binaryURL, checksumURL, installPath string, opts *DownloadOptions) error {
+	client := opts.httpClient()
+	assetName := filepath.Base(binaryURL)
+
+	tmpFile := installPath + ".tmp"
+	if err := downloadToFile(ctx, client, binaryURL, tmpFile); err != nil {
+		return fmt.Errorf("failed to download binary: %w", err)
+	}
+
+	if err := verifyFileChecksum(ctx, client, tmpFile, assetName, checksumURL); err != nil {
+		_ = os.Remove(tmpFile)
+		return fmt.Errorf("checksum verification failed: %w", err)
+	}
+
+	if err := os.Chmod(tmpFile, 0o755); err != nil {
+		_ = os.Remove(tmpFile)
+		return fmt.Errorf("failed to make binary executable: %w", err)
+	}
+
+	if err := os.Rename(tmpFile, installPath); err != nil {
+		_ = os.Remove(tmpFile)
+		return fmt.Errorf("failed to install binary: %w", err)
+	}
+
+	return nil
+}
+
+func downloadToFile(ctx context.Context, client *http.Client, url, path string) error {
+	req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
+	if err != nil {
+		return err
+	}
+
+	resp, err := client.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("download failed with status %d", resp.StatusCode)
+	}
+
+	out, err := os.Create(path)
+	if err != nil {
+		return err
+	}
+	defer out.Close()
+
+	_, err = io.Copy(out, resp.Body)
+	return err
+}
+
+func verifyFileChecksum(ctx context.Context, client *http.Client, filePath, assetName, checksumURL string) error {
+	expectedHash, err := fetchExpectedChecksum(ctx, client, assetName, checksumURL)
+	if err != nil {
+		return err
+	}
+
+	f, err := os.Open(filePath)
+	if err != nil {
+		return fmt.Errorf("failed to open file for checksum: %w", err)
+	}
+	defer f.Close()
+
+	h := sha256.New()
+	if _, err := io.Copy(h, f); err != nil {
+		return fmt.Errorf("failed to compute checksum: %w", err)
+	}
+
+	actualHash := hex.EncodeToString(h.Sum(nil))
+	if actualHash != expectedHash {
+		return fmt.Errorf("sha256 mismatch: expected %s, got %s", expectedHash, actualHash)
+	}
+
+	return nil
+}
+
+func fetchExpectedChecksum(ctx context.Context, client *http.Client, assetName, checksumURL string) (string, error) {
+	req, err := http.NewRequestWithContext(ctx, "GET", checksumURL, nil)
+	if err != nil {
+		return "", err
+	}
+
+	resp, err := client.Do(req)
+	if err != nil {
+		return "", fmt.Errorf("failed to download checksums: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return "", fmt.Errorf("checksums download failed with status %d", resp.StatusCode)
+	}
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return "", fmt.Errorf("failed to read checksums: %w", err)
+	}
+
+	for _, line := range strings.Split(strings.TrimSpace(string(body)), "\n") {
+		parts := strings.Fields(line)
+		if len(parts) == 2 && parts[1] == assetName {
+			return parts[0], nil
+		}
+	}
+
+	return "", fmt.Errorf("no checksum found for asset %s", assetName)
+}